1. Field of the Invention
The present invention relates to an optical transmission method and device, and in particular to an optical transmission method and device on a point side in a PON (Passive Optical Network) transmission system of a point-multipoint type.
2. Description of the Related Art
Recently, demands for the construction of an optical access network represented by FTTH (Fiber to the Home) have been accelerated in order to realize advanced information communication services, in which a PON transmission system has been regarded as a means for realizing a high-capacity data communication and making the network construction inexpensive.
FIG. 10 shows an arrangement of such a PON transmission system. In this system, an optical line terminal OLT on a point (station) side is mutually connected to optical network units ONUa, ONUb, . . . , ONUx (hereinafter, occasionally represented by a reference character ONU) on multipoint sides bundled by a coupler 20, based on a clock synchronization. The technology of controlling a plurality of optical network units ONUs by a single optical line terminal OLT is prescribed in ITU-T recommendations G.983 and G.984.
The optical line terminal OLT transmits, an initial setting information, transmission enabling phase information TPP and transmission enabling (allowable) duration information TPT to each optical network unit ONU through the coupler 20. In each optical network unit having received the transmission enabling phase information TPP and the transmission enabling duration information TPT, e.g. the optical network unit ONUa transmits (emits) light at a designated transmission enabling phase ta, the optical network unit ONUb similarly transmits light at a designated transmission enabling phase tb, and the optical network unit ONUx performs the light transmission at a designated transmission enabling phase tx.
The optical line terminal OLT monitors, as shown in FIG. 11, an optical signal transmitted from each optical network unit ONU around the transmission enabling phase designated per transmission line clock. For example, in the absence of a light transmission (emission) of the optical network unit ONUa in a phase tolerance PAR where phases for several clocks around the transmission enabling phase ta which the optical line terminal OLT has designated to the optical network unit ONUa are supposed to form the phase tolerance PAR as shown in FIG. 11, it is determined that no light is emitted (no-burst). The determination of the light transmission from the optical network unit ONU has not been performed in the outside of the phase tolerance PAR (NPAR), so that an optical network unit ONU abnormally emitting light has not been specified.
On the other hand, there are a method and system for notifying abnormal state of subscriber line transmission line which have a line terminal having a network monitoring device which manages a network state, notifies a state to a user upon detection of abnormality, and responds to an inquiry from the user, and a communication abnormality processor, in which an abnormal network unit number is notified to all of the network units upon detection of communication abnormality, and a network unit having an OAM processor, an ATM multiplexer/demultiplexer, a PON terminal portion, and an SEL, in which based on the abnormal network unit number from the line terminal, the state notification from the network monitoring device is connected to the user, a failure inquiry from the user is connected to the network monitoring device, a predetermined backup path is set up for the network unit which has a failure, and the network unit is connected to the user through the path and is connected to the network monitoring device inquired from the user (see e.g. patent document 1).
Also, there is a failure detection method in an optical transmission system in which a line terminal and an optical network unit are connected with a star coupler and an optical fiber transmission line, a passive double star communication or the like is performed by an optical signal, a compulsory emission instructing cell is branched by the optical fiber transmission line and the star coupler to be transmitted to the optical network unit from the line terminal individually, the optical network unit having received the compulsory emission instructing cell compulsorily emits light, and the line terminal receives the compulsorily emitted light, and based on the reception or non-reception in this case, a point where an individual failure of the optical fiber transmission line, star coupler, or the optical network unit has occurred is separately detected (see e.g. patent document 2).
[Patent document 1] Japanese Patent Application Laid-open No. 9-224037
[Patent document 2] Japanese Patent Application Laid-open No. 10-93483
While in the above-mentioned PON transmission system, the optical network unit transmits the optical signal based on the transmission enabling phase information from the optical line terminal, there has been a problem that since the optical signals from a plurality of optical network units are multiplexed by the coupler, in the presence of an abnormal optical network unit which transmits the optical signal at a phase other than that permitted by the optical line terminal, the optical signal collides with the light emission of other normal optical network unit, thereby preventing the communications.